ABSTRACT The primary objective of this proposal is to conduct a comprehensive study of bone safety with concurrent use of tenofovir (TDF)-based pre-exposure prophylaxis (PrEP) for HIV prevention and depot medroxyprogesterone acetate (DMPA) use for pregnancy prevention. Bone safety with the concurrent use of these agents is extremely relevant for young women in settings with high HIV burden and frequent use of DMPA, such as Uganda. As single interventions, both of these agents are known to reduce bone mineral density, a key marker for fracture risk.Bone fractures often result in lost wages and economic stability, severe consequences for young women. Furthermore, reductions in bone mass before the age of 25 can yield reduced peak bone mass, a frequent correlate of osteoporotic fracture later in life. Recent approvals of TDF-based PrEP from Sub-Saharan African drug authorities will accelerate its roll-out, making studies of its effects when added on top of other agents extremely timely and important for public health messaging. We hypothesize that there are direct links from concurrent TDF and DMPA use to bone loss through subclinical kidney injury or hypoestrogenism and will comprehensively assess markers of both mechanistic pathways. We will enroll a prospective cohort of 400 HIV- uninfected women ages 16-25 years initiating DMPA and condoms in Kampala, Uganda, and offer PrEP to all of them. Using radiologic, biochemical, and epidemiologic methods, the following specific aims will be addressed: 1) In a direct investigation of bone acquisition, we will use dual energy x-ray absorptiometry (DXA) to assess whether young women using TDF-based PrEP and DMPA concurrently attain lower peak bone mass over a 24- month period and have evidence of disrupted microarchitecture, relative to women using either agent singly or neither agent; 2) We will investigate whether young women concurrently using TDF-based PrEP and DMPA experience: 1) higher rates of bone turnover, a direct precursor to bone loss and 2) markers distinguishing two mechanistic pathways between TDF and DMPA use and bone mass: subclinical kidney injury and hypoestrogenism; and 3) Using mediation analysis, we will identify the degree to which the pathways through subclinical kidney injury and hypoestrogenism account for changes in bone density among women concurrently using TDF-based PrEP and DMPA. This study will be the first to comprehensively evaluate and model reductions in bone mass, changes in bone metabolism, and the pathways between bone and kidney function and bone and estrogen that are altered by the simultaneous initiation of PrEP and DMPA during a critical time point of bone maturation for young women.